Imo leveling up too quickly would get annoying. It'd be ~fine if you were just taking care of one character, but in BG3 (single-player) you have to level up all of your companions too. This takes some time - going into each character's level up page, making any relevant choices, etc. Leveling up should be an impactful thing that happens after you've really earned it, not an everyday occurrence.
I want a bit of time to get settled and comfortable with my current options (spells, abilities, etc) before having to level up again. Specifically, spending significant time at a certain level will help you understand which spells/abilities you need to add to your repertoire, and which spells are no longer useful and should be replaced on level-up.
As long as we level up often enough that each level doesn't feel stale/get boring, that's fine with me. No need to level quickly just for leveling's sake.
